This paper discusses angular aberration of the CMB (well known; see also astroph/0112457, a reference the authors seem to have missed). If an observer moves with respect to the CMB they see a dipolelike rotation of angles on the sky, leading to a magnification and demagnification in opposite hemispheres.
As it happens I've also been thinking about this the last week. Two points:
* Gravitational lensing of the CMB in general has a dipole convergence which is exactly the same as lowest order abberation; if you want to measure the primordial dipole (in principle) you have to get round this.
* I believe you can show that the frame in which the kinetic aberration vanishes is the same frame as the one in which the kinetic contribution to the dipole CMB vanishes, and coincides with the Newtonian Gauge (in linear theory). This seems to be an interesting physical identification of the CMB frame with the Newtonian gauge.
[astroph/0601559] Aberration of the Cosmic Microwave Background
Authors:  Scott Burles, Saul Rappaport 
Abstract:  The motion of the solar system barycenter with respect to the cosmic microwave background (CMB) induces a very large apparent dipole component into the CMB brightness map at the 3 mK level. In this Letter we discuss another kinematic effect of our motion through the CMB: the small shift in apparent angular positions due to the aberration of light. The aberration angles are only of order beta ~0.001, but this leads to a potentially measurable compression (expansion) of the spatial scale in the hemisphere toward (away from) our motion through the CMB. In turn, this will shift the peaks in the acoustic power spectrum of the CMB by a factor of order 1 +/ beta. For current CMB missions, and even those in the foreseeable future, this effect is small, but should be taken into account. In principle, if the acoustic peak locations were not limited by sampling noise (i.e., the cosmic variance), this effect could be used to determine the cosmic contribution to the dipole term. 
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